Industries of water electrolysis or alkali metal chloride aqueous solution electrolysis are heavy power-consuming type industries, and a variety of technology developments are being performed for energy saving. The method for saving energy thereof involves substantially reducing the electrolysis voltage which includes the theoretical decomposition voltage, the solution resistance, the ion-exchange membrane resistance, the anode overvoltage, the cathode overvoltage, and the like.
In particular, in relation to reducing the overvoltage, since the overvoltage value thereof is influenced by the catalyst material of the electrode, the morphology of the electrode surface, or the like, much research and development has been previously performed regarding the improvement thereof. With regard to an ion-exchange membrane process for sodium chloride electrolysis, as a result of active research and development being performed on the reduction of the anode overvoltage, dimensionally stable electrodes with low anode overvoltages and excellent durabilities (for example, the DSE electrode (registered trademark) manufactured by Permelec Electrode Co.) were completed, and these are already being utilized in a wide range of fields, including the sodium chloride electrolysis industry.
On the other hand, there have also previously been many propositions in relation to electrodes for hydrogen generation, or so-called activated cathodes, for reducing the cathode overvoltage (for example, Patent Document 1).
Generally, a means for decreasing the hydrogen overvoltage represents an increase in activity of a supported catalyst and an increase in the reactive specific surface area. For improving the activity, a high activity catalyst of a metallic compound, a metallic alloy, an oxide, or compounds thereof which has a specific composition is supported on a conductive base material, and the increase in specific surface area is improved as a result of the support method thereof. Examples of the main support methods include: the electroplating method that electrodeposits the catalyst component from the vessel in which the active component and the metal salt has been dissolved; the dispersal plating method that electrodeposits the catalyst component by electrophoresis from the vessel in which the active material has been dispersed in a metal salt solution; the spray coating method that thermally sprays the melt state catalyst material directly onto a base material; and the thermal decomposition method that coats and bakes a metal salt solution and the like.
Conventionally, as an electrode that can decrease the hydrogen overvoltage of an iron cathode of approximately 400 mV to 150 to 200 mV, for example, a method has been disclosed in which an alloy layer of a transition metal of iron, cobalt, and nickel and one of tungsten and molybdenum is supported on the surface of a conductive base material by the electroplating method (Patent Document 1).
Furthermore, an electrode has been disclosed that supports a material containing, in addition to a combination of nickel and one of iron, cobalt, and indium, an organic compound such as an amino acid, a carboxylic acid, or an amine, on the surface of the conductive base material by the electroplating method (Patent Document 2).
However, in these electrodes, since it is necessary to make the supported material very thick, it is easy for distortions of the electrode and peeling of the supported material to occur as a result of the plating stress. Also the activity of these base metals are low. Therefore, the improvement of the activity by alloying the base metals alone was insufficient as an effect that decreases the hydrogen overvoltage.
Furthermore, an electrode in which an alloy layer containing nickel and molybdenum is supported by the arc ion plating method has been disclosed (Patent Document 3); however, despite the initial hydrogen overvoltage being sufficiently low, there was a problem in the hydrogen overvoltage rising during long electrolysis operations, or the so-called durability.
On the other hand, an electrode for hydrogen generation has been disclosed which includes a three-component alloy consisting of: nickel and/or cobalt; a component selected from aluminum, zinc, magnesium, and silicon; and a noble metal such as platinum (Patent Document 4).
This electrode is disclosed for the purpose in which the component selected from aluminum, zinc, magnesium, and silicon is eluted and removed from the alloy including the aforementioned three components and a Raney nickel and/or Raney cobalt catalyst is utilized in the electrode for hydrogen generation. As a result of the addition of the noble metal component at a minute amount of less than 0.4 by mole ratio, the deterioration of the electrode activity due to the decomposition of nickel and/or cobalt to nickel hydroxide or cobalt hydroxide is prevented; thereby, the improvement in durability is achieved.
However, since in this electrode, the hydrogen overvoltage is reduced by increasing the specific surface area of the nickel and/or cobalt, as well as a process that removes components from the catalyst being necessary, it is necessary to thicken the supported material to several tens of microns to several hundreds of microns. Therefore, there were problems such as the production costs being very high. In Patent Document 4, it is disclosed that there is no reduction effect in the hydrogen generation overvoltage, even when the amount of the noble metal component was set to be 0.4 or more by mole ratio.
Furthermore, electrodes for hydrogen generation have been conventionally proposed which include a mixture of Ni or a Ni alloy and a platinum group metal and the like. For example, in Patent Document 5, an electrode for hydrogen generation including at least one cathode active material selected from among platinum group metals and/or platinum group metal oxides which is dispersed in Ni or a Ni alloy, that is to say, an electrode for hydrogen generation has been proposed in which a mixture of Ni or a Ni alloy and at least one cathode active material selected from among platinum group metals and/or platinum group metal oxides is coated.
As an active nickel coating which mainly contains Ni or an alloy thereof, an active material is selected from a porous nickel which is obtained by coating Ni and a sacrificial metal and then eluting the sacrificial metal, and an alloy and/or a mixture which includes Ni and other metal and/or other compound. On the other hand, as other metals that are coated together with Ni, many materials have been proposed such as Fe, Mo, Co, W, Al, Zn, Sn, Mg, Ti, platinum group metals, and their oxides.
In Patent Document 5, as an electrode for hydrogen generation with a particularly low hydrogen overvoltage and an excellent durability, a coating has been exemplified that contains active nickel in which platinum group metal particles or platinum group oxide particles such as platinum black, ruthenium black, ruthenium oxide, and the like are mixed and dispersed in Ni.
However, as is disclosed in the Examples of Patent Document 5, with regard to an electrode for hydrogen generation including Ni containing ruthenium oxide particles which is one of the most superior examples mentioned above, even in the case in which the current density is low such as 0.20 A/cm2 (2 kA/m2), the hydrogen generation potential is −0.98 V vs. NHE. When this value is converted to a hydrogen overvoltage, it is approximately 120 mV, and the hydrogen overvoltage performance is insufficient. That is to say, each of the electrodes for hydrogen generation proposed in Patent Document 5 which are achieved by coating various metals or the oxides thereof such as a platinum group metal together with Ni or a Ni alloy were not satisfactory in the overvoltage performance.
In addition, mixtures and complexes of platinum group metal oxides and oxides of Ni and the like has been conventionally proposed to use. For example, in Patent Document 6, a method for producing an electrode which includes a mixed oxide or a complex oxide containing a platinum group metal oxide and a Ni oxide has been proposed. In the method, the mixed solution of the platinum group metal compound and metal compound of Ni and the like are coated and dried, and then a heat treatment is conducted under sufficient conditions for oxidizing the metal compound, that is to say, a heat treatment under an oxidizing flow of air or oxygen and at a high temperature.
In Example 3 of Patent Document 6, an electrode for hydrogen generation is disclosed in which oxides of platinum, nickel, and ruthenium coated, and the electrode is produced by coating a mixed solution of chloroplatinic acid, nickel chloride, and ruthenium chloride on a nickel base material and drying the solution, and then subjecting to a thermal decomposition at 470 to 480° C. The potentials measured at 0.31 A/cm2 (3.1 kA/m2) are disclosed in Ex. 3 of Table I. When the potentials are converted into overvoltages using the actual absolute reversible voltages in the thermodynamic calculations disclosed in Patent Document 6, the overvoltage of the first cycle is 42 mV which is sufficiently satisfactory; however, the overvoltage rises together with the electrolysis progress, and the hydrogen generation overvoltage of the sixth cycle is 87 mV, and the hydrogen generation overvoltages of the eleventh or more cycles are 97 mV. Accordingly, in the case in which a current density of 5 kA/m2 or more is applied, the overvoltage is predicted to be 100 mV or more at the very lowest. Therefore, there was a problem that should be improved.
On the other hand, in addition to the above, electrodes for hydrogen generation have been conventionally proposed in which a plurality of noble metal group elements and base metal elements have been combined. For example, in Patent Document 7, electrodes for hydrogen generation has been proposed in which a noble metal precipitate containing one noble metal or a mixture or an alloy of two or three or more of noble metals, or a precipitate containing the above-mentioned noble metal precipitate and one or two or more of base metals such as Ni is deposited onto a conductive base material of Ni and the like.
However, with regard to these electrodes for hydrogen generation, it is known that they possess a problem in that they are susceptible to poisoning as a result of impurities such as iron in the electrolyte (Patent Document 8 and Patent Document 9).
In this manner, electrodes for hydrogen generation which support platinum and have a low overvoltage have been conventionally proposed. However, electrodes for hydrogen generation which support platinum are sensitively susceptible to the effects of poisoning due to small amounts of iron ions which are present in the electrolyte, and even when the iron ion concentration is a trace concentration of 1 ppm or less, the hydrogen overvoltage rises. Consequently, further improvements are being examined in regard to the utilization in the electrolysis industry of alkali metal chloride aqueous solutions and the like in which iron ions tend to be mixed into the electrolyte (Patent Document 8).
Accordingly, wide examinations are being performed for the purpose of preventing poisoning due to iron ions in the electrolyte, and a variety of propositions have been performed.
The applicant has previously investigated the relationship between the precipitation of iron on the cathode and the iron ions in the catholyte in the case in which a hydrogen generation cathode having a low hydrogen overvoltage is used for the electrolysis of an alkali metal chloride aqueous solution. The applicant has found that the precipitation of iron can be prevented in the case in which the iron ion concentration in the catholyte is 0.5 ppm or less. Then, a method for electrolyzing an alkali metal chloride aqueous solution was proposed in which a low hydrogen overvoltage cathode is used, and the electrolysis is performed while maintaining the iron ion concentration in the catholyte to 0.5 ppm or less (Patent Document 9).
As a result of such an invention, the electrodes for hydrogen generation which sensitively receive the effects of poisoning due to iron ions can be utilized in the electrolysis industry of the alkali metal chloride aqueous solution and the like. However, in order to execute the proposition of Patent Document 9, it is necessary to use a material such as high Ni type stainless or Ni in at least areas that are anodically polarized amongst portions that make contact with the catholyte, and is also necessary to flow an anticorrosion current at the time when the electrolysis is stopped. Therefore there were problems to be improved upon from an economic viewpoint.
Furthermore, a method was examined for removing iron from an electrode for hydrogen generation in which the overvoltage has risen as a result of the iron ions, and propositions have been performed in which the iron is removed from an electrode for hydrogen generation that has a deteriorated hydrogen overvoltage due to the precipitation of iron, and the electrode is reused.
For example, a method for removing iron that has been precipitated on the cathode surface was proposed which includes bringing the cathode surface into contact with a liquid medium that reacts with the iron precipitated on the surface thereof and solubilizes the same (Patent Document 10).
As a result of using this method, electrodes for hydrogen generation in which the overvoltage has risen due to the iron ions can be reused; however, in order to execute this proposition, there was a need to frequently stop electrolysis, and it was not possible to stably operate for continuous long periods. Accordingly, in this case, there were also problems to be improved upon from an economic viewpoint.
Furthermore, attempts have been conventionally widely performed to impart characteristics in which iron ions are hard to adhere to the electrode for hydrogen generation itself, or the performance does not deteriorate even when iron ions adhere.
For example, electrodes for hydrogen generation were proposed which support a catalyst containing platinum, ruthenium, and at least one of gold and silver, or a catalyst that further contains particles of an organic polymer on a conductive base material (Patent Document 11).
With regard to the aforementioned electrode for hydrogen generation, the rise in the overvoltage is very small even when iron ions are present in the catholyte, and it is an electrode for hydrogen generation that certainly possesses excellent characteristics in regard to reducing the energy consumption of electrolysis of alkali metal chloride aqueous solutions.
However, platinum, ruthenium, gold, and silver are all expensive materials, and in the case in which polytetrafluoroethylene is included in the electrode for hydrogen generation, it becomes even more expensive. Accordingly, in this case, there were also problems to be improved upon from an economic viewpoint.
On the other hand, an electrode for hydrogen generation has been proposed in which a catalyst including platinum and cerium oxide is used (Patent Document 12).
With regard to the electrode for hydrogen generation including this platinum and cerium oxide catalyst, the overvoltage is low, the effect of iron ions is restricted, and it exhibits excellent characteristics as an electrode for hydrogen generation for electrolysis of alkali metal chloride aqueous solutions.
Furthermore, a proposition has been performed in which an intermediate layer including a nickel oxide is provided between the catalyst containing platinum and cerium oxide and a base material (Patent Document 12), and examinations are being performed in order to further improve cost problems and the like.
As described above, various electrodes for hydrogen generation and methods for utilizing the electrodes for hydrogen generation have been conventionally proposed for the purpose of reducing the power consumption of the electrolysis of water or alkali metal chloride aqueous solution. However, with regard to the conventional electrodes for hydrogen generation, an electrode for hydrogen generation has not still been obtained which satisfies both of the hydrogen overvoltage characteristic, the resistance characteristic to poisoning due to iron ions in the catholyte, together with a sufficient durability in the industrial utilization in which stop-and-start control is conducted, and industrially satisfactory characteristics.
Patent Document 1: Japanese Examined Patent Application, Second Publication No. Sho 40-9130
Patent Document 2: Japanese Patent Publication No. 3319370 (Examples)
Patent Document 3: Japanese Patent Publication No. 3358465 (Examples)
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. Sho 59-25985 (Claims, from line 13 in the upper right column on page 2, to line 16 in the lower right column on page 2, Examples)
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. Sho 54-110984 (lines 10 to 16 in the upper left column on page 3, from line 17 to the last line in the lower left column on page 3)
Patent Document 6: Japanese Unexamined Patent Application, First Publication No. Sho 59-232284 (claim 3, from line 10 in the lower left column on page 6, to line 5 in the lower right column on page 6, from line 4 in the lower right column on page 7, to line 19 in the lower right column on page 10, Examples)
Patent Document 7: Japanese Unexamined Patent Application, First Publication No. Sho 57-23083 (Claims, from line 17 in the lower left column on page 4, to line 9 in the lower right column on page 4)
Patent Document 8: Japanese Unexamined Patent Application, First Publication No. Sho 64-8288 (Claims, from line 15 to the last line in the lower left column on page 2)
Patent Document 9: Japanese Unexamined Patent Application, First Publication No. Sho 60-56082 (Claims, from line 19 in the upper left column on page 2, to line 16 in the lower left column on page 2, from line 18 in the upper left column on page 3, to line 4 in the lower left column on page 3)
Patent Document 10: Japanese Unexamined Patent Application, First Publication No. Sho 60-59090 (Claims)
Patent Document 11: Japanese Unexamined Patent Application, First Publication No. Sho 63-72897 (Claims)
Patent Document 12: Japanese Unexamined Patent Application, First Publication No. 2000-239882 (Claims, paragraph 0004, paragraph 0006)